1. Field of the Invention
The present invention relates to a filter device, and more particularly, a filter device constituted of an acoustic wave element such as a surface a coustic wave (SAW) element having a resonance characteristic in the VHF and UHF bands.
2. Description of the Related Art
The filter device is widely used in the communication field. In particular, the filter device using a surface acoustic wave (SAW) resonator, or a surface acoustic wave (SAW) filter (hereinafter generically termed SAW element), is largely used for mobile communication such as mobile telephone, because of being able to be compact in size, with a low loss.
In recent years, mobile telephone becomes to have high functionality, and TV broadcast has become possible to view by a mobile phone having a mounted TV tuner. Particularly in Japan, types of mobile phones capable of receiving “one-segment” broadcast, a type of digital TV broadcast, have been increasing.
For example, when a TV tuner is mounted on a WCDMA mobile phone (transmission signal of 830-840 MHz) in the Japanese domestic telephone operating companies, the frequency band I of one-segment broadcast is 470-840 MHz, as shown in FIG. 1. However, when viewed from the TV tuner for one-segment broadcast, WCDMA transmission signal II (830-840 MHz) is a cause of trouble such as crosstalk.
Accordingly, a band pass filter capable of passing only the one-segment broadcast frequency band I is necessary on the one-segment broadcast TV tuner side.
As an element for realizing the band passfilter, compactness is a necessary condition when taking into consideration to mount on portable equipment. As a compact filter element, the use of an acoustic filter such as surface acoustic wave (SAW) device may be considered. However, since the frequency band I of the one-segment broadcast is a wideband of 470-770 MHz, it is difficult to realize a bandpass filter of such wideband by use of the SAW device.
Incidentally, in FIG. 1, the frequency band III is a frequency bard for use in another mobile telephone system, and therefore has no relation to the WCDMA portable equipment in the present example.
Accordingly, the use of a band elimination filter for attenuating the WCDMA transmission signal II (830-840 MHz) may be considered. In such the case, the high frequency (770 MHz) side of the one-segment TV broadcast signal is exceedingly close to the low frequency (830 MHz) side of the WCDMA mobile telephone signal. Therefore, the band elimination filter is required to have a steep cut-off characteristic.
Meanwhile, usually, the filter device is also required to have balanced input and output because an element having balanced terminals, such as a frequency converter, an amplifier, or the like, is connected before and after the filter device. As a conventional method to realize the band elimination filter, a method using a ladder-type structure (as disclosed in the U.S. Pat. No. 5,854,579) shown in FIG. 2 has been known, which has, however, unbalanced input and output.
Also, as shown in FIG. 3, there have been devised methods for realizing balanced input and output using a ladder type (as in the Japanese Unexamined Patent Publication Nos. Hei-11-346142, 2000-22493 and 2001-7680).
The band elimination filter using a ladder-type structure shown in FIG. 2 has unbalanced input and output. When the above band elimination filter is connected to an element having balanced terminals, it is not possible to connect directly, and an unbalance-balance converter is necessary.
In contrast, the filter shown in FIG. 3 is of ladder-type structure having balanced input and output. However, the number of resonator elements becomes large, which does not meet the requirement to make a device element compact.
Considering the above-mentioned points, the inventors of the present application have been studying a variety of band elimination filter types having balanced input and output with a reduced number of configuration elements, and come to obtain a novel band elimination filter structure that meets the aforementioned requirement and has a desired steep cut-off characteristic.